This application relates generally to medical implant devices and their production, specifically relating to the process of manufacturing a polymer tissue and/or bone fixation device, preferably made of a resorbable polymer. The invention more particularly concerns a method of manufacturing a resorbable bone fixation device (e.g., screw, rod, pin, etc.) by forcing a provided polymer slug or billet into a mold while the polymer is in a glass transition state, wherein the manufacturing process creates a near final shape of varying diameter and zones with variable degrees of alignment of the polymeric molecular structure and tailored mechanical properties (e.g., higher strength).
Common techniques utilized in the past for the production of shaped polymer materials have included, machining (e.g. milling, turning, etc.), injection molding, and extrusion. Machining a desired shape from a generic slug or billet often results in excessive waste, as the amount of material that is trimmed or cut off in making the final product will be much greater than the amount removed during final machining of a molded or formed polymer material that is shaped nearly to final form. For example, in machining a screw shape, having a head and a threaded body portion, from a slug or billet in the shape of a cylinder, material must be removed to arrive at the diameter of the head. Subsequently, more material must be removed to arrive at the desired diameter for the threaded body portion. This extensive machining creates a great amount of chips or cut dust as waste of the material that is machined off.
Excessive waste of raw material is especially problematic in devices constructed of relatively expensive polymers, such as bioabsorbable polymers and medical grade polymers, as costs are elevated due to the loss of the material, or additional costs are incurred in recapturing and recycling the material. A need exists for a manufacturing technique that results in higher productivity and higher yield than machining.
Injection molding is a process in which a polymer is heated to a highly plastic state and forced to flow under high pressure filling a mold cavity, where it solidifies. Melt molding processes result in a material having a relaxed orientation or molecular arrangement of the polymer as it cools, and typically does not impart great strength values, such as those required for tissue and/or bone fixation treatments suitable for implantation through surgical techniques (e.g., orthopedic and traumatology applications).
It has long been known that the production of a polymer material having an aligned orientation (i.e., not relaxed) of the polymer molecules or structure typically results in a stronger material. This correlation has been discussed in the prior art, for example see U.S. Pat. Nos. 3,161,709; 3,422,181; 4,282,277; 4,968, 317; and 5,169,587, where it is described that polymer materials may be drawn or extruded to cause the orientation of a semi-crystalline or crystalline polymer structure to become substantially aligned, thereby increasing the mechanical strength of the material.
As discussed in U.S. Pat. No. 4,968,317 issued to Tormala et al., the prior art of using melt molding techniques such as injection molding and extrusion to make resorbable polymer implants results in strength values that are typical of thermoplastic polymers. It is known that the strength and modulus values may be increased by creating a reinforced composite (i.e., incorporating reinforcing fibers), however to achieve satisfactorily large strength values with reinforced composites as implants, the implant must necessarily be large in order to accommodate the stresses placed upon it.
As is known, and is further described by Tormala et al., a technique for the processing of polymer material may utilize mechanical deformation, such as drawing or hydrostatic extrusion, to alter the orientation of the molecular structure of crystalline structure and amorphous structure to a fibrillar state, in order to yield higher strength and elastic modulus values. Tormala et al. describe drawing the material through the extrusion process, resulting in an extruded material that is at least partially fibrillated as the polymer molecules and molecular segments are aligned along the drawing direction. Tormala et al. in U.S. Pat. No. 6,383,187 describe a resorbable screw made of the material described in the U.S. Pat. No. 4,968,317 patent. A need exists for a fibrillar material that may be created in varying cross-sections and diameters, in order to minimize the amount of machining required to finish the product. A further need exists for an implantable device having variable states or degrees of alignment of the polymer molecules. This may be accomplished by manufacturing or processing a material that is formed to final part geometry or near final part geometry of a device or implant, thereby reducing the need for final machining, and also obtaining increased mechanical strengths for implant applications.
In U.S. patent application 2003/0146541, Nakamura et al. describe a press molding process for the manufacture of a resorbable polymer bonejoining device having molecular orientation. The described process requires imparting the existing molecular orientation, preferably by stretching the primary article along the long axis, then providing the oriented primary article for press molding of the screw head and shank threads. The press molding as applied to the polymer material allows the molecular orientation of the primary article to be substantially maintained. Nakamura et al. do not describe a process for creating a device having variable cross section and variable states of alignment of the polymer molecules, wherein the process of manufacturing the areas with varying cross-sections imparts an increased orientation of the polymer molecules.
In U.S. patent application 2003/0006533, Shikinami et al. disclose a twice-forged resorbable polymer material, wherein the polymer molecular orientation is altered by each of the forging processes to create “orientation along a large number of reference axes having different axial directions”. The forging steps applied to the polymer result in the orientation of the polymer molecules to create a room temperature flexible material, capable of withstanding repeated bending without breaking. U.S. patent application 2003/0006533 does not describe a polymer material that is shaped into varied cross-sections and possessing varied zones of polymer orientation.
In U.S. Pat. No. 6,232,384, Hyon discloses a resorbable bone fixation material comprising a resorbable polymer, hydroxyapatite and an alkaline inorganic compound, wherein the bone fixation material is made by the process of providing a melt with the aformentioned components, molecularly orienting the melt through a molding or extension process and extending and orienting the chain molecules of the polymer. Preferably the molding process is performed through ram or hydrostatic extrusion. Hyon does not describe an implantable material having varied cross-section and varied zones of polymer orientation.
In U.S. Pat. No. 6,503,278, Pohjonen et al. disclose an implantable surgical device made from a resorbable, non-crystalline (i.e., amorphous) polymer. The amorphous material described by Pohjonen et al. is molecularly oriented and reinforced by mechanical deformation. Pohjonen et al. do not describe a polymer implant material having zones of variable states of alignment of the polymer molecules and varying cross section of the material.
In U.S. Pat. No. 5,431,652, Shimamoto et al. disclose a high strength polymer material that is hydrostatically extruded through a die under pressure to reduce voids and to form a resorbable polymer material that retains at least 85% of its strength after 90 days implantation. The material described in the Shimamoto et al. patent does not result in a polymer implant material or implant with complex geometry or variable shape other than the cross section of the die exit, nor does Shimamoto et al. arrive at or describe variable states of alignment of the polymer molecules.
In U.S. Pat. No. 6,511,511, Slivka et al. disclose a polymer implant that is either porous or non-porous, where the material has been reinforced by the addition of oriented fibers. The Slivka devices are made by precipitating the polymer out from a solvent solvating the polymer. The precipitation of the polymer causes a gel formation, which may then be handled and placed in a mold. Slivka et al. do not describe a polymer implant having variable shape and variable states of alignment of the polymer molecules.
The prior art described does not disclose a polymer implantable device having an orientation of the polymer molecules, wherein the shaping process creates zones of varying cross section and orientation.
It is the intent of this invention to overcome these and other shortcomings of the prior art.